Rosenergoatom said the amount of fuel loaded into the reactor reached “minimum critical mass” – the minimum amount of fuel required for a sustained nuclear chain reaction – on 21 June 2014.

The company said final preparations and procedures are being followed to obtain approval from the regulator for the reactor to achieve first criticality, which is expected “in the coming days”.

After the unit achieves first criticality, it will be connected to the grid and begin to produce commercial electricity at a minimum power level.

The unit’s power level will then be increased in stages until the unit is brought into commercial operation. This is expected to happen in 2015, Rosenergoatom said.

Beloyarsk-4 is a 789-megawatt sodium-cooled fast reactor of the BN-800 design. It burns mixed uranium-plutonium fuel.

Rosenergoatom said the BN-800 unit will be used for development of fast reactor technology with a closed fuel cycle. This could lead to a 50-fold increase in the amount of energy extracted from natural uranium, substantially expanding the nuclear industry’s fuel base, Rosenergoatom said.

There is already one commercially operational reactor at the Beloyarsk nuclear station, the Beloyarsk-3 BN-600 fast reactor unit. Two other units, both AMB-100 light water graphite reactor blocks commissioned in the 1960s, have been permanently shut down. Construction of a fifth fast reactor unit with a capacity of about 1,200 megawatt is in the planning stage.